1. Field of the Invention
The present invention relates to an electrochromic organic compound and an electrochromic device using the same.
2. Description of the Related Art
Various materials have been reported as an electrochromic (hereinafter may be abbreviated as “EC”) material in which optical absorption properties (colored state and light transmittance) of a material is changed by an electrochemical oxidation-reduction reaction. A material using a metal oxide such as WO3 is known as an inorganic EC material, but the material has a problem when a device having a large area is produced because a film-forming method is limited to deposition or the like.
Incidentally, conductive polymers such as polythiophene and polyaniline, organic low-molecular compounds such as viologen and oligothiophene and the like are known as an organic EC material. Here, examples of the organic low-molecular EC compounds include a viologen derivative which is a cathodic compound that is colored by reduction and an oligothiophene derivative which is an anodic compound that is colored by oxidation.
These organic low-molecular EC compounds transmit visible light in the neutral state because the compounds each have a shorter π-conjugated chain length than a conductive polymer and have absorption in an ultraviolet region. Further, these organic low-molecular EC compounds absorb visible light in the oxidized state (state of an anodic compound) or in the reduced state (state of a cathodic compound). This is because the wavelength region that absorbs light is a visible light region since the conjugated chain length of the compound in the oxidized state or the reduced state is longer than that of the compound in the neutral state.
That is, the organic low-molecular EC compound is characterized in that it is bleached in the neutral state and colored in the oxidized state or the reduced state.
Japanese Patent Application Laid-Open No. H09-120088 discloses a viologen derivative which is colored in the reduced state, and International Publication WO2013/099523 and CHAIN-LENGTH DEPENDENCE OF ELECTROCHEMICAL AND ELECTRONIC-PROPERTIES OF NEUTRAL AND OXIDIZED SOLUBLE ALPHA, ALPHA-COUPLED THIOPHENE OLIGOMERS, GUAY, J; KASAI, P; DIAZ, A et al., Chemistry of Materials, 1992, vol. 4, No. 5, pp. 1097-1105 disclose an oligothiophene derivative which is colored in the oxidized state.
Incidentally, an EC device using an organic low-molecular EC compound has high transparency in the bleached state. Examples of the EC device using an organic low-molecular EC compound that is known include a sandwiched-type device in which the EC compound is dissolved in a solvent such as propylene carbonate and the solution is sandwiched with a pair of electrodes.
In the solution-type EC device using an organic low-molecular EC compound, the organic low-molecular EC compound can be dissolved in a solvent at high concentration in order to increase the optical density (decrease transmittance) in the colored state.
However, an oligothiophene derivative disclosed in CHAIN-LENGTH DEPENDENCE OF ELECTROCHEMICAL AND ELECTRONIC-PROPERTIES OF NEUTRAL AND OXIDIZED SOLUBLE ALPHA, ALPHA-COUPLED THIOPHENE OLIGOMERS, GUAY, J; KASAI, P; DIAZ, A et al., Chemistry of Materials, 1992, vol. 4, No. 5, pp. 1097-1105 has a low solubility in a solvent, and the stability of the compound in the case of repeating an oxidation-reduction reaction is low. Further, although the thiophene derivative disclosed in International Publication WO2013/099523 has high durability against repetition of an oxidation-reduction, its solubility in a polar solvent used in an EC device is not so high.
On the other hand, although the viologen derivative (cathodic compound) described in Japanese Patent Application Laid-Open No. H09-120088 has high solubility in a polar solvent used in an EC device resulting from the molecular structure thereof, it has been a problem that the stability of the compound in the case of repeating an oxidation-reduction reaction is low.